1. Field of the Invention
This invention relates to an infrared detector which converts infrared rays to an electrical signal, and more particularly to the electrode structure for a multi-element infrared detector.
2. Description of the Related Art
It is known that all objects radiate infrared rays unless the temperature thereof is reduced to absolute zero. Therefore, when infrared rays from an object are detected by an infrared detector, the temperature thereof or temperature differences therein can be identified, and an image of the object can be obtained if the object is scanned two-dimensionally.
A single-element infrared radiation detector is easy to fabricate, however, it takes several seconds to scan a single frame on a display because the optical system including a mirror must be moved in both horizontal and vertical directions.
In order to shorten the required scan time for a single frame, a linear (one-dimensional) array of detector elements has been proposed and developed. This type of device requires an optical scan in only one direction, either horizontally or vertically, resulting in a significant curtailing of the scan time and thus facilitating the production of images nearly on a real time basis. Detectors having two-dimensional arrays, i.e., matrix arrays of detector elements make the optical scan mechanism unnecessary and are most desirable.
PbS (lead sulphide), PbSe (lead selenide), InSb (indium antimonide), PbSnTe (lead tin telluride), HgCdTe (mercury cadmium telluride), etc., are well known photoconductive materials for detecting infrared rays. Among these materials, HgCdTe has extremely high sensitivity in wavelengths ranging between about 3 and 14 .mu.m, depending on the mole fraction x of CdTe (cadmium telluride) to HgTe (mercury telluride) in the ternary compound semiconductor Hg.sub.1-x Cd.sub.x Te. When x is about 0.2, the HgCdTe material has very high performance characteristics in detectivity (D.sup.*) and responsivity (Re) at wavelengths of about 12 .mu.m, and further such material has low noise characteristics and a small time constant. Therefore, HgCdTe is a very sensitive and effective material for detecting real images of objects or at near room temperature.
Generally, infrared detectors using the above photoconductive materials have better characteristics when the detector elements are cooled to a low temperature. For example, HgCdTe or InSb infrared detectors are cooled to about 77.degree. K. by liquid nitrogen during operation.
There are many problems to be solved in fabricating multi-element infrared detectors. One such problem is that uniform characteristics for a plurality of detector elements are difficult to achieve when the number of detector elements is increased. The reasons for this are as follows. During the fabrication of multi-element infrared detector elements, a first end of each detector element is connected to a common terminal of a power source to provide a bias current of a specific amount to flow through each of the detector elements. Conventionally, a common metal line connecting such first ends of the detector elements is formed on the photoconductive layer so as to run parallel to the detector element array, and such line is connected further to a terminal pad at the end portion of the array. This type of structure presents the problem that the requisite thick common metal lines are difficult to form on photoconductive layers.
But when thin lines are used instead, the resultant increase in the resistance of the detector circuit causes a reduction in the sensitivity of the device for detecting temperature differences, and further, because electrical resistance from the terminal pad to each of the detector elements differs depending on the distance between the pad and the detector, the sensitivity of each detector element is different because the bias current flows through each of the detector elements at a different rate depending on the distance of the element from the terminal pad.
A uniform electrode structure having low electrical resistance will solve the above problem if it can be easily formed. In order to form a thick metal layer on a substrate on which a photoconductive layer has previously been formed, it is necessary to first solve the problems presented by the deposition and patterning of the metal layer, because the photoconductive layers generally used are very sensitive to the wet etching solutions that have conventionally been used in the past.